Etymology

Belegost (beleg + ost) was a Sindarin translation of the original Dwarvish name Gabilgathol and both mean "Great City". Unlike other names of the Silmarillion, the text also gives us an English rendering, which was possibly from Westron: Mickleburg.[7]Mickle is a root meaning "big"; see also Michel Delving.

The city's Khuzdul name Gabilgathol contains the elements gabil "great" and gathol "fortress".

↑J.R.R. Tolkien, "Eldarin Hands, Fingers & Numerals and Related Writings — Part Two" (edited by Patrick H. Wynne), in Vinyar Tengwar, Number 48, December 2005, p. 24 ("...which had certainly been founded long ago ... before the coming of the exiled Noldor, probably before the Eldar of the Great Journey ever reached Beleriand")